There are known methods for delivering a composition to a substrate. These techniques may vary depending on the nature of the substrate to which a composition is to be applied, and whether or not the composition is required to penetrate the surface of the substrate. Many of these known methods require complex machinery and also active pressure systems which require a lot of power input. There may be a number of other disadvantages of known methods. The following background discussions outline, by way of example, techniques used to deliver compositions to lignocellulosic substrates such as freshly felled logs or processed lumber.
Lignocellulosic material, and more specifically debarked logs or sawn lumber, largely because these are of biological origin, are prone to attack by organisms such as bacteria, insects, nematodes and a variety of fungi including decay and staining fungi. Such attack reduces the service life of logs or lumber extracted therefrom, degrades the appearance of the logs or lumber, and reduces the service life of such materials with resultant cost of replacement or potential hazard due to failure.
Logs, or lumber, when freshly cut, generally are not contaminated internally by fungi. However, as soon as the wood substrate is exposed by either bark damage or by cutting, fungal spores attach to any exposed surface and begin growing by utilising the available carbohydrates and resin acids as a food source. A number of fungal species including Ophiostoma species are known to grow rapidly into the substrate and quickly move beyond the control of readily available fungicides. There is also a window of opportunity for pest organisms to colonise such substrates between the period of felling and the time at which pesticidal treatments are applied. This infection by pest organisms prior pesticidal treatment may be referred to as “pre-infection” and is a significant problem for the lumber industry.
To mitigate infection by biological pests methods have been developed to treat these substrates with a variety of chemicals by various physical processes. Unfortunately, these processes may be considered to have one or more disadvantages. Some such processes and their disadvantages are noted herein below.
Lignocellulosic substrates are complex structures including lumber cells interconnected by pits which include a membrane otherwise acting as a valve system when the tree is living. Such cells and cell interconnections offer impedance to the flow of preservative into the substrate. This is more particularly so when the substrate is dry because the pit membranes aspirate, that is they collapse to either side of the pit and effectively seal it shut. Drying of the substrate however is important prior to treatment with preservative because space is required within which to place the preservative. Consequently a large number of variations of vacuum/pressure processes have been developed to overcome this impedance thus allowing flow of a composition (for example a biocide or preservative composition) throughout the entire substrate to ensure total protection.
Several such processes include those of: 1) Rueping: Pre-pressure with gas followed by pressure with preservative or chemical solution; 2) Lowry: Pressure impregnation with preservative or chemical solution; and 3) Bethel: Vacuum followed by pressure impregnation with preservative or chemical solution. These processes are described in “Industrial Timber Preservation”, 1979, J G Wilkinson, Associated Business Press.
The Rueping process applies pre-pressure with gas prior to treatment with preservative fluids. This pre-pressure with gas fills the cells with a compressible medium such that after treatment with fluid the gas will expand forcing out any surplus fluid. However this can result in ongoing kickback of preservative contaminated fluid which may be hazardous and which kickback fluid may contain extractives which will interfere with preservative chemistry.
The Rueping and Lowry processes retain gas within the void spaces within the substrate. Thus, the impregnation process requires pumps to force fluid into the substrate against the back pressure of the gases in the voids.
The Bethel process removes all gases from the cells by application of a vacuum which cells then become completely filled with preservative fluid. This method has the disadvantage that lumber is completely filled with aqueous fluid which can not be sucked out again. Accordingly, the lumber takes considerable time to dry.
It is noted that some of these processes necessitate impregnation of up to 700 litres of chemical solution per cubic metre of lumber, in for example softwoods.
These systems also have the disadvantage that preservative or chemical composition is recycled back and forth between the pressure vessel and the storage tanks. This can lead to contamination of the solution by lumber extractives. This can cause problems. For example, in cases where preservatives contain hexavalent chromium, lumber extractives in the preservative composition may cause a chemical imbalance. Recycling of compositions can also be a problem where ammoniacal preservatives are used because ammonia can volatilise causing precipitation of other components resulting in hazardous emissions. Use of alternative chemicals which are not necessarily preservatives also suffer from these problems.
There are examples of processes which attempt to alter the properties of some substrates such as lumber by impregnation with polymeric materials. Plant and equipment for such processes is very expensive. Large steel pressure vessels are required which must sustain positive pressures of 600 pounds per square inch, or vacuum of nearly minus 15 pounds per square inch. To assist these processes it is necessary to have large, high volume, high pressure pumps and, high volume, high vacuum pumps. Not only are these expensive, they must operate in quite corrosive chemical environments, which leads to costly repair and maintenance. Since the treatment plant is large and the pressure vessel may have a capacity of 30 cubic metres, it is necessary to have similar sized storage vessels to contain the operating fluid. Due to the size of the plant equipment and the risk of preservative spillage it is necessary to house the plant in a contained building. Accordingly, this type of facility is very capital expensive to build and operate.
A critical issue of all the above processes is the size of the operation and more significantly the need to store large volumes of hazardous chemicals.
Various other methods have been developed to treat lumber with various compositions. For example: U.S. Pat. No. 3,964,863 describes a method which uses a pressure vessel for treatment of lumber in which the pressure of the impregnating fluid is increased isostatically in a controlled manner causing the impregnating fluid to be distributed throughout substantially all of the material; U.S. Pat. No. 4,303,705 describes a method of treatment of lumber with water-borne preservatives, such as CCA salts, in which the preservatives are forced into the lumber under pressure and the water-borne lumber treatment materials are held within the lumber under pressure until they are deposited as by precipitation or chemical affixation; and, U.S. Pat. No. 4,433,031 describes a method of impregnating lumber with a preservative wherein the treatment is carried out by exposing the lumber, while submerged in the treating solution, to one or more cycles of reduced pressure (i.e. vacuum) and elevated pressure.
Additional examples include: U.S. Pat. No. 4,466,998 which describes the use of water-borne preservatives to impregnate lumber by the empty-cell pressure impregnation method without the precipitation of water-borne salts such as chromium, copper and arsenic when lumber sugars enter the treating solution during the kickback or pressure release phase of the empty-cell cycle. The lack of precipitation is obtained by maintaining the preservative salts solution at a temperature between about 40 degrees Fahrenheit and about 70 degrees Fahrenheit; and U.S. Pat. No. 5,970,624 describes a lumber impregnation method which comprises a process to heat lumber, a process to place the heated lumbers under a vacuumed pressure, a process to immerse the heated lumbers in processing agents under the vacuumed pressure, and a process to restore the pressure of the lumbers immersed in the processing agents under the vacuumed pressure to the air pressure, and to immerse the lumbers in the processing agents under the air pressure.
Further examples include: the method of U.S. Pat. No. 5,871,817 which details immersing dry lumber in a preservative sustained by a small hydrostatic head can achieve effective impregnation; the method described in U.S. Pat. No. 6,235,403 in which in one aspect lumber is impregnated with a waterborne preservative such as CCA at elevated temperature and pressure; and the method of NZ 235036 which involves impregnating a lignocellulosic substrate to modify its physical properties (namely to increase density or harden the substrate) using traditional methods of vacuum pressure impregnation.
U.S. Pat. No. 5,871,817 describes a method which eliminates the need for large pressure and vacuum pumps. However, the plant is still complex and expensive and the process requires storage of moderate volumes of preservative chemicals. This process relies on a limited hydrostatic head to impregnate the lumber by placing the dried lumber submersed under preservative solution into a controlled depth dip tank. In such case the dried lumber at ambient temperature is submerged in a preservative solution.
As mentioned above, pre-infection of lumber with pest organisms, such as fungi, is a major problem for the lumber industry. A recent trend has been to formulate mobile biocides that have the ability to penetrate or diffuse into the substrate. Unfortunately these are often toxic and more significantly they may be readily washed from the substrate surface by rain if exposed to such immediately after application. Accordingly, other methods have been developed in attempts to effectively deliver biocide compositions to the lignocellulosic material.
Whilst modern formulations have been developed which give a degree of penetration radially into the substrate surface, this movement is not large. Since the fungi of most concern are able to grow up to 1.5 mm per day under optimum conditions it is currently necessary to treat with chemical within approximately 48 hours of exposing fresh substrate surface. In most situations this is impractical and therefore an alternative method of control or remediation is needed.
A particular difficulty in effectively treating lumber for or from pre-infection is that the lumber needs to be treated soon after felling. In this condition, the lumber contains a lot of moisture, as opposed to processed or milled lumber which will have been substantially dried. The moisture content of this type of lumber may impede the absorption of a biocide composition into it to sufficient depth to prevent or remediate growth of pest organisms, particularly fungi.
Attempts have been made to use vacuum and pressure impregnation processes (such as those described above) to force formulations into wet or saturated organic substrates to a depth which would control fungal and other pest attack. Unfortunately, ineffective penetration and excessive processing costs may result.
As mentioned previously herein, there are a number of methods of sterilisation of logs arid lumber. Both hot water and steam at ambient pressure have been used to condition logs prior to conversion to veneer. Similarly steam at elevated pressure has been used commercially in poles and lumber to reduce moisture and to open pathways such as ray parenchyma such that pressure impregnation with preservatives can be facilitated. These methods however, may suffer from a number of problems.
Specific examples of methods which have been used in attempts to control pre-infection of lumber are provided below. As with the examples provided above, such methods may suffer from one or more problems.
U.S. Pat. No. 5,505,240 describes the sterilisation of debarked logs using hot water treatment. The process involves water in which the debarked logs are immersed being heated indirectly using steam coils which are submerged in the water.
U.S. Pat. No. 5,447,686 describes sterilisation of logs using steam or hot water. The process may involve the addition to the steam of a chemical comprising a biocide. This has some serious limitations in that volatile biocides must be used and which could pose severe risk to health. Additionally, in the case of hot water containing a biocide there remains a large volume of biocide containing fluid and this poses an environmental risk particularly as it will also accumulate extractives from the logs and ultimately require disposal. More serious disadvantages of this process may be difficulty in having the biocide adequately penetrate the substrate and that prevention of cracking, checking and decay is not achieved.
McLean describes the principles, processes and times required to achieve sterilisation and conditioning of substrates; MacLean, J. D. 1952. Preservative treatment of wood by pressure methods; US Department of Agriculture, Agriculture Handbook No. 40. McLean however does not anticipate the consequence of moisture loss or the need to apply subsequent prophylactic treatments.
U.S. Pat. No. 4,978,501 describes a process for sterilisation of mushroom casings using radio frequency energy. In this process casings which are typically comprised of lignocellulosic materials, are passed through an electromagnetic field. If sufficient power is applied for long enough this will result in sterilisation of the substrate. Since mushroom casings will be used immediately in relatively sterile environments there is no need to prevent re-infection.
U.S. Pat. No. 6,014,819 describes amelioration of fungal degrade by exposure of hardwood logs to steam. The process involves the moisture content of the substrate being maintained. Addition of biocides may occur during the heating process. This process may suffer from the negative features associated with the process of U.S. Pat. No. 5,447,686.
The above description and examples highlight the problems associated with known methods for treatment or impregnation of organic substrates with compositions such as preservative compositions. Problems may include one or more of: necessity to use expensive plant and processing equipment; large plant and processing equipment; extended processing and/or treatment times; excessive environmental impact; excessive energy requirements; waste of treatment composition; ineffective delivery of composition to target zones within the substrate being treated. Such problems may also exist in relation to treating organic substrates with other compositions.